Free-standing, 2D gold nanosheets (AuNS) offer broad
potential
applications from computing to biosensing and healthcare. Such applications,
however, require improved control of material growth. We recently
reported the synthesis of AuNS only ∼0.47 nm (two atoms) thick,
which exhibited very high catalytic activity. The synthesis is a one-pot,
seedless procedure in which chloroauric acid is reduced by sodium
citrate in the presence of methyl orange (MO). In this study, we use
spectrophotometric analysis and TEM imaging to probe AuNS formation
and optimize the procedure. Previously, we suggested that MO acted
as the confining agent, directing two-dimensional growth of the gold.
Here, we provide the first reported analysis of the HAuCl4 and MO reaction. We show that MO is rapidly oxidized to give 4-diazobenzenesulfonic
acid, indicating that a complex interplay between HAuCl4, MO, and other reaction products leads to AuNS formation. Time-resolved
studies indicate that synthesis time can be significantly reduced
from over 12 to 2–3 h. Decreasing the reaction temperature
from 20 to 4 °C improved AuNS yield by 16-fold, and the catalytic
activity of the optimized material matches that obtained previously.
Our elucidation of AuNS formation mechanisms has opened avenues to
further improve and tune the synthesis, enhancing the potential applications
of ultrathin AuNS.
Inorganic nanoparticles have long been applied as catalysts and nanozymes with exceptional rate constants arising from their large surface areas. While it is understood that high surface area-to-volume ratios and low average atomic coordination are responsible for their exceptional catalytic properties, these facets remain under exploited in the design of gold nanoparticle catalysts and nanozymes. Here we have developed 3D, 2D, and quasi-1D gold nanoparticles for use as catalysts in reducing 4-nitrophenol by sodium borohydride. Each morphology was characterised with transmission electron microscopy and UV-Vis absorption spectroscopy, while the highest catalytic activity was achieved when the perimeter-to-surface area, or amount of ‘edge’, was maximised. The particles were then applied as nanozymes in modular nano-composite hydrogels. Independent hydrogel tiles containing either the substrate or catalyst were bonded in stacks, which allowed reagent transport across their interface for the colourimetric detection of hydrogen peroxide. This work presents novel insight into the catalytic activity of low-dimension nanoparticles and their potential application in nanozyme-based diagnostic devices.
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